Grain-refined aluminum-iron alloys



United States Patent 9 3,026,197 GRAIN -REFINED AL -IRON ALLOYS Jacob H. Schrarnm, Bayonne, N.J., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Feb. 20, 1959, Ser. No. 798,313 5 Claims. (Cl. 75-124) This invention relates to aluminum-containing iron base alloys and in particular it relates to a method, and products produced therefrom, of grain refining such alloys.

The excellent magnetic properties and oxidation resistance of aluminum-containing iron base alloys, together with their acceptable tensile properties at elevated temperatures and the absence of critical alloying elements, has drawn attention to these alloys for commercial applications. As with any new alloy system, problems have been encountered in the development and utilization of these alloys for commercial operations. The aluminumirons with an aluminum content on the order of to 18 weight percent are brittle and require special fabrication techniques to reduce them to a tape or sheet thickness as well as to other shapes. For example, in the Nachman Patent No. 2,801,942, there is disclosed what is now known as a warm rolling procedure which is used to produce an elongated microstructure in aluminum-irons and thereby impart a degree of low temperature ductility. It may be noted upon studying that patent, that close temperature control must be maintained in practicing the invention.

Part of the difficulty in commercializing these alloys has been attributed to their characteristic coarse grain structure. It is therefore evident that the ability to provide these materials with grain refinement and grain size control would materially enhance their commercialization.

It is an object of the present invention to provide a method of grain refining aluminum-containing iron base alloys.

It is another object of this invention to provide iron base alloys and articles formed therefrom that include aluminum within the magnetic range and which are characterized by a refined grain structure.

In accordance with the present invention, these objects are attained by including in aluminum-containing iron base alloys a small but efiective amount of zirconium and boron to characterize the resulting alloy with a refined grain structure. In this manner the coarse grains inherent in this type of ferritic alloy are avoided and a structure is produced which is characterized by a uniformly small grain size.

The advantages of the present invention are attained upon adding zirconium and boron to iron base alloys containing aluminum. Experiment has shown that both of these elements must be used to provide the desired results. Generally, the zirconium and boron are added, relative to one another, in substantially a 1:2 atomic weight ratio, though an excess of either may be used if desired. Based on the weight of the alloy, about 0.05 to 0.5 percent of zirconium and 0.01 to 0.1 percent of boron are generally used.

The alloy systems with which the present invention can be practiced are iron base alloys that contain aluminum, on a weight basis, in amounts of at least 3 percent and suitably about 6 to 18 percent. While grain refinement may be attained in these iron base alloys throughout the range of aluminum contents, it is of most significance with those alloys that are inherently brittle. Accordingly, the preferred alloys contain aluminum in the range of about 10 to 18 weight percent. In addition to aluminum and iron, the alloys can include such alloying additions as chromium, titanium, molybdenum and similar alloying ice elements that are added to aluminum-irons to develop particular characteristics in the resulting alloy.

The alloys can be prepared in the same manner as aluminum-iron alloys are presentely made, For example, they may be melted in air or in a controlled atmosphere, such as a vacuum or in the presence of a gas inert to the alloy such, for instance, as helium, argon and the like and including mixtures of such gases. Suitable procedures are disclosed in the patent literature. See, for example, the United States patent to Morgan et al. Number 2,804,387, and the Nachman patent above referred to. In applying the present invention with those procedures, zirconium and boron in the desired amounts are generally added to the iron melt after it is refined and alloyed with the aluminum. Sufiicient time is provided to permit the zirconium and boron to be dispersed throughout the melt. Thereafter, the resulting melt is cast.

The advantages of the present invention are attained with both the as-cast alloys and worked alloys as will be demonstrated in connection with the specific examples discussed hereinafter. The significance of the resulting grain refinement brought about in this invention is most evident in comparing the workability of alloys. For example, with two alloys that differ only in that one is made in accordance with the present invention whereas the other conforms to the prior art, the orging of these materials promptly shows the improved and superior workability brought about by the grain refinement. Thus in an alloy free of the zirconium and boron addition, it is found that the very first stages of forging generally result in a tendency of those coarse grained ingots to crack. This is especially so at the higher aluminum contents and with ternary and quaternary additions. However alloys prepared as in this invention are readily worked under the same conditions that bring about cracking in the other alloys, and high yields of products are obtained. In addition, the resulting materials evidence smooth edges and -a far better surface than the alloys that do not contain zirconium and boron as in the present invention.

As illustrating the invention, two ingots were prepared with compositions which differed essentially only in that one contained zirconium and boron in accordance with the present invention while the other was free from those elements. Melting conditions, pouring, and mold temperatures used were essentially identical for the two ingots. The nominal analysis of the reference alloy (ingot 1) was 15.8 weight percent of aluminum and the remainder iron, while the analysis, by weight, of the other ingot (ingot 2) was 14.4 percent of aluminum, 0.1 percent of zirconium, 0.02 percent of boron and the remainder iron. The ingot cross-sections were 2 /2 x 2 /2 inches. Samples of the ingots were polished and etched in the conventional manner and macrographs of their structures were taken. It was'readily apparent upon inspection of these two macrographs that the presence of zirconium and boron, as in this invention, results in a greatly refined grain structure as compared to the usual structure of these alloys.

Portions of ingots l and 2 were forged at temperatures between 900 and 1050" C. In the very first stages of forging ingot l, a tendency to crack was noticed. No cracks developed under the same conditions of forging in the alloy of ingot 2, thereby demonstrating the practical advantage of grain refinement in accordance with this invention.

Other samples of the alloys of ingots l and 2 as well as additional alloys were hot forged to strips that were 2 /2 inches wide and A inch thick. After heating the strips to a temperature within the range of 600 to 800 C., they were reduced to a 25 mil thickness by rolling between cold rolls. After each pass of the rolls an intermediate anneal was conducted on each strip by heating to the temperature range of 600 to 800 C, When the 25 mil thickness was reached, the resulting strips were visually examined. In each instance it was noted that the alloy free form the zirconium and boron (ingot 1) exhibited cracked or rough edges while the boronand zirconium-containing alloy showed smooth edges and the better surface.

To illustrate the advantage in rolling as a consequence of this invention, a test was made whereby alloys of ingots, hereinafter identified as ingots 3 and 4, prepared and rolled as just described were reduced to a thickness of 75 mils. At this point samples were annealed at 700 C. Photomiorographs were then made as before. The nominal composition of the alloy of ingot 3 was 11.2 percent of aluminum, 5.86 percent of chromium and the remainder iron while that of ingot 4 was 11.2 percent of aluminum, 4.68 percent of chromium, 0.2 percent of zirconium, 0.05 percent of boron and the remainder iron. Comparing ingots 3 and 4 it Was noted that the zirconium and boron can easily be observed as a heterogeneous phase in the ferritic matrix elongated in the rolling direction.

Photomicrographs were also made on strip samples, of each of the four ingots above identified, sectioned parallel to the rolling direction. A sample of each strip was annealed at 1200 C. for an hour, and micrographs of each were made. A second sample was annealed at 1000 C.; micrographs of this material were also made. Micrographs were made to show the alloy strips as cut from the sample without further annealing, and are therefore in the as-rolled condition. These micrographs showed dramtically the effect of the use of zirconium and boron in producing and maintaining a refined grain structure. It is expected that grain coarsening will occur with ferritic alloys upon annealing, and that result was observed in the micrographs. In each instance the grain size becomes larger, progressing from the as-rolled condition to the annealed states. However, the zirconium and boron additions (ingots 2 and 4) markedly suppressed the degree of coarsening which occurs. It is apparent, therefore, that grain refinement can be maintained Without regard to processing conditions.

In other tests that have been made on the usefulness of this invention it was demonstrated that the presence of zirconium and boron had a marked effect in avoiding grain boundary cracking that is a frequent consequence in the severe quenching of metals. In this test, strips 5 inches long, inch wide and mils thick were prepared from alloys difliering only in that one contained zirconium and boron as in this invention whereas the other Was free from these additions. Strips of this nature were annealed at 1100 C., 1000 C. and so on down to a lower temperature in 100 stages. After each annealing stage they were water quenched. The alloys free from zirconium and boron cracked after the first, or a very few, quenches while the zirconiurnand boron-containing alloys did not crack even after 12 quenches. The cracks in this instance were determined by microscopic examination as well as by measurement of the electrical-resistance of the specimens.

In addition to the workability advantages above noted, alloys of this invention surprisingly evidence better oxidation resistance at these aluminum concentrations than the comparison alloys free from zirconium and boron. In one series of oxidation resistance tests the following cycle Oxygen Pick-Up. Grams per S quare Centimeter Aluminum Zirconium Boron Iron Remainder Remainder It may be noted that the decrease in oxygen pick-up as a consequence of this invention is on the order of a factor of four.

From the foregoing discussion and data it is readily apparent that this invention provides effective grain size control, oxidation resistance and superior workability in ferritic alloys containing aluminum in significant proportions.

In accordance with the provisions of the patent statutes, the invention has been explained and there has been described what is now believed to be its best embodiment. However, it should be understood that the invention can be practiced otherwise than as specifically described.

I claim as my invention:

1. An alloy characterized by a refined grain structure and resistance to grain coarsening upon annealing, consisting essentially of, by weight, from 6 percent to 18 percent of aluminum, up to the order of 5.86 percent chromium, 0.05 to 0.5 percent of zirconium, 0.01 to 0.1 percent of boron, and the remainder iron.

2. A cast alloy characterizezd by a refined grain structure and resistance to grain coarsening upon annealing, consisting essentially of, by weight, 10 to 18 percent of aluminum, 0.05 to 0.5 percent of zirconium, 0.01 to 0.1 percent of boron, and the remainder iron.

3. A cast alloy in accordance with claim 2 in which said zirconium and boron are present in an atomic weight ratio relative to one another of about 1 to 2.

4. An alloy sheet characterized by a refined grain structure and resistance to grain coarsening upon annealing, consisting essentially of, by weight, 10 to 18 percent of aluminum, 0.05 to 0.5 percent ofzirconium, 0.01 to 0.1 percent of boron, and the remainder iron.

5. An alloy sheet in accordance with claim 4 in which said zirconium and boron are present in an atomic weight ratio relative to one another of about 1 to 2.

References Cited in the file of this patent UNITED STATES PATENTS 1,641,752 Flintcrmann Sept. 6, 1927 1,833,723 Ruder Nov. 24, 1931 1,850,953 Armstrong Mar. 22, 1932 FOREIGN PATENTS 184,840 Great Britain Aug. 21, 1922 OTHER REFERENCES Cornelius et al.: Archiv fiir das Eisenhuttenwesen, vol. 13, No. 12, June 1940, pages 539-542. Published by Verlag Stahleisen m.b.H., Dusseldorf, Germany. 

1. AN ALLOY CHARACTERIZED BY A REFINED GRAIN STRUCTURE AND RESISTANCE TO GRAIN COARSENING UPON ANNEALING, CONSISTING ESSENTIALLY OF, BY WEIGHT, FROM 6 PERCENT TO 18 PERCENT OF ALUMINUM, UP TO THE ORDER OF 5.86 PERCENT CHROMIUM, 0.05 TO 0.5 PERCENT OF ZIRCONIUM, 0.01 TO 0.1 PERCENT OF BORON, AND THE REMAINDER IRON. 